In recent years, there have been glowing demands in the field of production of various electronic devices requiring fine processing such as semiconductor elements towards the devices progressively increased in density and integration. This makes very severe the required performances for the photography techniques for realizing refined patterns. Contributing to this refining technique are photoresists increased in resolution and exposure light having a shortened wavelength.
In general, the resolution (Res) of the optical system can be represented by the Rayleigh equation, namely Res=k.multidot..lambda./NA (wherein k is a process factor, .lambda. is a wavelength of an exposure light source, and NA is the number of openings of a lens). This equation shows that the width of a reproduced line can be decreased to resolve a fine pattern (namely high resolution can be obtained) by shortening the wavelength at the time of exposure. Surely, the exposure wavelength has been sifted to the g-line (436 nm) and the i-line (365 nm) of a high pressure mercury lamp with a decrease in the minimum width of the reproduced line, and the production of the devices by use of the KrF excimer laser beam (249 nm) has been studied. For further fine processing, the use of an excimer laser beam having a shorter wavelength, particularly ArF (193 nm), has a good prospect.
Looking at photoresists exposed to shortwave light, high integration in multilayer resist systems utilizing surface lithography, not in monolayer resist systems which have previously been used in the industrial production, is also studied. However, it still suffers from the problem of complicated processes which have prevented the practical application of the multilayer resists.
In the case of excimer lasers including KrF excimer lasers, it is generally considered that the life of gases is short, and that the cost performance of the lasers is required to be improved because exposure devices themselves are expensive.
Responding to this are so-called chemical amplification type resists becoming the main current in KrF excimer laser exposure applications. In the chemical amplification type resists, acids are generated from photo acid generators existing in catalytic amounts in the systems by exposure, and protective groups of alkali-soluble groups of binders or low molecular weight compounds are eliminated with the catalytic amount of acids by the catalytic reaction to ensure discrimination of the solubility in alkali developing solutions. In the chemical amplification type resists, the acids generated by the photocatalytic reaction are catalytically utilized, so that an increase in sensitivity is expected.
In general, the chemical amplification system resists can be roughly divided into three classes, commonly called as a 2-component system, a 2.5-component system and a 3-component system. In the 2-component system, a photo acid generator is combined with a binder resin. The binder resin is a resin having a group which is decomposed by the action of an acid to enhance the solubility of the resin in an alkali developing solution (which is also referred to as an acid decomposable group) in its molecule. The 2.5-component system contains a low molecular weight compound further having an acid decomposable group in addition to such a 2-component system. The 3-component system contains the photo acid generator, the alkali-soluble resin and the above-mentioned low molecular weight compound.
However, when the wavelength of exposure light becomes short, a new problem is encountered. That is, in the photoresists, raw materials good in transparency to shortwave light is poor in resistance to dry etching. On the other hand, there is the problem that raw materials good in resistance to dry etching is poor in transparency. The compatibility of the resistance to dry etching and the transparency is basically the problem of the performance of the binder resins contained in photoresist layers.
The binder resins include novolak resins and poly(p-hydroxystyrene). The novolak resins are widely utilized as alkali-soluble resins for i-line resists, and the poly(p-hydroxystyrene) resins are used as base polymers for KrF excimer laser resists. These produce no problem as long as long-wave light is used. However, different therefrom, the use of shortwave light rises a problem. In particular, the above-mentioned resins have high optical density within the wavelength region of 170 nm to 220 nm. It is therefore actually difficult to directly use these resins as with the conventional methods. Accordingly, the development of resins high in light transparency and resistance to dry etching has been looked forward to.
One of the general solutions to this problem is a method of introducing, for example, an alicyclic hydrocarbon moiety into the resin. There is also a method of utilizing a naphthalene skeleton, one of the aromatic compounds. In particular, various reports disclose that the introduction of alicyclic hydrocarbon moieties fulfills demands for both light transparency and resistance to dry etching. For example, it is described in Journal of Photopolymer Science and Technology, 3, 439 (1992).
On the other hand, what to select as the acid decomposable group contained in the resin is important, particularly, because it affects the sensitivity and resolution of the resist and further the aging stability.
The acid decomposable groups for protecting carboxylic acid groups, which have hitherto been mainly reported, include tertiary alkyl esters such as t-butyl esters and acetal esters such as tetrahydropyranyl esters and ethoxyethyl esters. However, the t-butyl ester groups have the drawback that the ability of being eliminated with the generated acids is low, resulting in a lowering of the sensitivity. Conversely, the tetrahydropyranyl esters and the ethoxyethyl esters have a large problem with the aging stability because of their easy decomposition at ordinary temperatures.
Further, JP-A-5-346668 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") has proposed to use 3-oxocyclohexyl ester groups as the acid decomposable groups. However, they are not necessarily satisfactory in sensitivity.
As described above, for the acid decomposable groups for protecting the carboxylic acids which satisfy the sensitivity and the aging stability of the photoresists at the same time, how to design the acid decomposable groups has been not necessarily clear.
Furthermore, in order to decrease the width of the reproduced line to resolve the fine pattern in the resulting resist pattern, not only the above-mentioned characteristics but also sufficient adhesion of the fine pattern obtained to a substrate is required. Even if the fine pattern is obtained, insufficient adhesion thereof sometimes causes separation thereof.
On the other hand, in photofabrication processes of lithography, the production of semiconductors for ICs and the production of circuit substrates for thermal heads, semiconductor wafer, glass, ceramic or metal substrates are coated with photoresists to a thickness of 0.5 .mu.m to 2.5 .mu.m by spin coating or roller coating, followed by heating and drying. Then, circuit patterns are printed thereon through exposure masks with active light such as ultraviolet rays, and baked after exposure if necessary, followed by development to form images of the resists. Further, pattern processing can be performed on the substrates by etching using the images as masks.
In many cases, alkali-soluble resins and photosensitive materials are generally used in combination as positive type photoresist compositions, and especially, compositions comprising novolak type phenol resins and naphthoquinone-diazide compounds in combination are used. The positive type photoresists comprising the novolak resins and quinonediazide compounds have advantageous characteristics such as high resistance to plasma etching, the prevention of dissolution of the naphthoquinonediazide compounds, the disappearance of the ability of preventing the compounds from being dissolved, associated with carboxylic acids by light irradiation, and an improvement in alkali solubility of the novolak resins as a result thereof. Accordingly, a number of the photoresists have been developed and have come in practice, and sufficient results have been obtained for processing of a line width up to about 0.8 .mu.m to about 2 .mu.m.
However, integrated circuits have been progressively increased in their integration degree, and therefore processing of ultrafine patterns having a line width of a half micron or less has become necessary in the production of semiconductor substrates for very large scale integrated circuits. For fulfilling this necessity, the wavelength of light used in exposure devices employed for photolithography becomes progressively shorter, and now, of the far ultraviolet rays, the use of excimer laser light having a short wavelength (such as XeCl, KrF or ArF) has been studied.
In the pattern formation of lithography in this wavelength region, the absorption of the novolak resins and the naphthoquinonediazide compounds in the far ultraviolet region is so strong that light becomes difficult to arrive at the bottom portions of the resists. Accordingly, only patterns low in sensitivity and tapered are obtained. The novolak-naphthoquinonediazide resists are therefore insufficient.
One means for solving this problem is chemical amplification system resist compositions described in U.S. Pat. No. 4,491,628 and European Patent 249,139. The positive type chemical amplification system resist composition is a composition designed so as to produce an acid in an exposed area by irradiation of active light such as far ultraviolet rays and to change the solubility of the area irradiated with active light and an area not irradiated therewith in a developing solution by a reaction using this acid as a catalyst, thereby forming a pattern on a substrate.
Examples of such resins which are used in combination with the photo acid generators and changed in solubility with the acids include acetal or O,N-acetal compounds (JP-A-48-89003), orthoesters or amidoacetal compounds (JP-A-51-120714), polymers having acetal or ketal groups on main chains (JP-A-53-133429), enol ether compounds (JP-A-55-12995), N-acyliminocarboxylic acid compounds (JP-A-55-126236), polymers having orthoester groups on main chains (JP-A-56-17345), tertiary alkyl ester compounds (JP-A-60-3625), silyl ester compounds (JP-A-60-10247) and silyl ether compounds (JP-A-60-37549 and JP-A-60-121446). These compounds exceed 1 in quantum yield in principle, so that they exhibit high photosensitivity.
Similarly, the systems which are stable at room temperature, but are decomposed in the presence of acids by heating to be solubilized in alkalis include, for example, combined systems of photo acid generators described in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240 JP-A-63-250642, Polym. Eng. Sci., 23, 1012 (1983), ACS. Sym., 242, 11 (1984), Semiconductor World, November, 91 (1987), Macromolecules, 21, 1475 (1988) and SPIE, 920, 42 (1988), and esters of tertiary or secondary carbon (for example, t-butyl or 2-cyclohexenyl) or carbonic ester compounds. These systems also have high sensitivity and can be effective for the above-mentioned shortening of the wavelength of light sources, because of their lower absorption in the far ultraviolet region, compared with the novolak resin/naphthoquinonediazide systems.
In particular, there are proposed resist compounds comprising hydroxystyrene polymers particularly low in photoabsorption when the 248-nm light of a KrF excimer laser is used, into which acetal groups or ketal groups are introduced as protective groups. Examples thereof are described in JP-A-2-141636, JP-A-2-19847, JP-A-4-219757 and JP-A-5-281745. Besides, similar compositions having t-butoxycarbonyloxy groups or p-tetrahydropyranyloxy groups as acid decomposable groups are proposed in JP-A-2-209977, JP-A-3-206458 and JP-A-2-19847. However, these compositions substantially have the disadvantage of low sensitivity caused by too high absorbance when an ArF laser is used as a light source. Accompanied thereby, they further have the problems of deterioration of pattern profiles and lack of focus permissibility, so that many improvements are required.
As the photoresist compositions for the ArF light source, photoresist compositions in which (meth)acrylic resins smaller in absorption than partially hydrogenated hydroxystyrene resins are combined with compounds generating acids with light are proposed, for example, in JP-A-7-199467 and JP-A-7-252324. Above all, JP-A-6-289615 discloses resins in which organic groups of tertiary carbon are attached by ester linkages to oxygen of a carboxyl group of acrylic acid, and JP-A-7-234511 discloses copolymer resins comprising structural units in which dimethyl-substituted tertiary carbon groups are attached by ester linkages to oxygen of a carboxyl group of acrylic acid, and structural units in which alicyclic groups are attached by ester linkages. These resins have photoabsorptive aryl groups in carbon substituent groups, or are poor in acid decomposability although improved in light permeability to the ArF light source. Thus, resins exhibiting characteristics sufficiently satisfactory to the object have not been obtained yet.
Here, also with respect to the photo acid generators used in the positive type chemical amplification resists as described above, the prior art is described. The known photo acid generators include N-imidosulfonates, N-oximesulfonates, o-nitrobenzylsulfonates and pyrogallol trimethanesulfonate. Further, as the agents high in photolysis efficiency and excellent in image forming properties, sulfoniums and iodoniums are known. As counter bases thereto, perfluoro Lewis acid bases such as PF.sub.6.sup.-, AsF.sub.6.sup.- and SbF.sub.6.sup.-, and further a trifluoromethanesulfonic acid anion and a toluenesulfonic acid anion are known. Furthermore, from the viewpoint of improving solvent solubility, benzene-sulfonic acid, naphthalenesulfonic acid and anthracene-sulfonic acid each having one straight-chain alkyl group or alkoxyl group are also disclosed. However, all of them are not sufficiently overcome in drawbacks such as contamination with counter anion elements and thinning of resist patterns with time from exposure to heating treatment, and more improvements in sensitivity and resolution are desired.